Non-aqueous secondary batteries such as a lithium ion secondary battery are characterized by a high energy density and thus have been widely used as a power source for portable equipment such as a portable telephone and a notebook personal computer. Recently, large lithium-ion secondary batteries such as an electric car battery and a stationary storage battery also come into widespread use.
By the way, non-aqueous secondary batteries for use as a power source for a portable telephone and the like are desired to have a higher capacity, improved storage characteristics, and improved charge and discharge cycle characteristics, and to have excellent convenience. Particularly, the non-aqueous secondary batteries are required to have both a high capacity to extend the duration of devices to be used and large current characteristics that enable charge and discharge at large current.
In order to increase the capacity of non-aqueous secondary batteries, for example, materials such as silicon (Si) and tin (Sn) capable of absorbing and desorbing as much lithium (Li) as possible are gaining attention as negative electrode active materials, in place of carbonaceous materials such as graphite that have been adopted in conventional lithium-ion secondary batteries. Patent Documents 1 and 2 reported that especially a material represented by General Composition Formula SiOx having a configuration in which Si ultra-fine particles are dispersed in the matrix of SiO2 have excellent load characteristics, in addition to the above characteristics.
Patent Document 3 discloses a positive electrode active material containing nickel (Ni), manganese (Mn), cobalt (Co), and another substituent element M in a specific ratio, wherein an atomic ratio of the substituent element M with respect to Ni, Mn and Co on the surface of the particle is larger than an average atomic ratio of the substituent element M with respect to Ni, Mn and Co in the entire particle. The positive electrode active material containing Ni as disclosed in Patent Document 3 has a larger capacity than LiCoO2, and hence is expected to further increase the capacity of lithium-ion secondary batteries.
Meanwhile, in order to enhance large current characteristics of non-aqueous secondary batteries, the resistance of an electrode should be reduced and the diffusibility of lithium ions should be enhanced in an electrode mixture layer. However, when the porosity of the electrode mixture layer is increased to enhance the diffusibility of lithium ions in the electrode mixture layer, the contact resistance between a current collector and the electrode mixture layer or between constituent particles such as active materials in the electrode mixture layer becomes large, and the impedance of the electrode is increased. Hence, large current characteristics cannot be enhanced as expected.
In order to enhance such large current characteristics of non-aqueous secondary batteries, for example, Patent Document 4 discloses a configuration for reducing the contact resistance between the current collector and the electrode mixture layer, in which a conductive layer containing electron conductive carbon fine particles is formed on a current collector made of a metal foil that is used in a conventional lithium-ion secondary battery